1. Field of the Invention
This invention relates to an improved process for the production of clear sodium silicate solutions from sand and sodium hydroxide under hydrothermal reaction conditions, followed by filtration.
2. Statement of the Related Art
Of the alkali metal silicate solutions known as "waterglass", sodium silicate solutions, known generally as soda waterglass, are the most commonly used for industrial purposes. Soda waterglasses generally have a solids content of from about 30 to 40% by weight and a ratio by weight of SiO.sub.2 to Na.sub.2 O of 3.3 to 3.4:1.
In general, soda waterglasses are industrially produced by melting quartz sand and soda together at 1,400.degree. to 1,500.degree. C. in suitable furnaces with evolution of CO.sub.2. The melt which solidifies on cooling to form lump glass is then dissolved in water in another process step carried out under pressure at elevated temperature and the solution obtained is optionally filtered, depending upon the quality requirements.
By subsequently mixing solutions such as these with sodium hydroxide, it is possible to obtain sodium silicate solutions richer in alkali with a lower ratio by weight of SiO.sub.2 to Na.sub.2 O. However, this process is very extravagant both in terms of plant and in terms of energy demand, incurring correspondingly high energy and plant costs, especially where it is desired to obtain waterglasses particularly rich in alkali with a ratio by weight of SiO.sub.2 to Na.sub.2 O of the order of 2:1, as required for various technical applications and, above all, for the synthesis of zeolites.
Historically, Van Derburgh reacted silica and alkali with superheated steam in a cylindrical pressure vessel with mechanical stirring, producing solutions (see U.S. Pat. Nos. 28,540; Re. 1,297; Re. 1,674; and Re. 1,675).
Another possible method for producing soda waterglasses is based on the fusion of sand by means of aqueous sodium hydroxide under hydrothermal conditions. Although the principles of this hydrothermal process have been known for some time (see German Pat. No. 244,779 and Chemical Engineering 5, 76 (1962)), processes of this type have only recently acquired any commerical significance due, on the one hand, to their unsatisfactory volume/time yields and, on the other hand, to the considerable difficulties involved in separating and filtering the highly viscous reaction solution.
According to Belgian Pat. No. 649,739, the difficulties involved in working up the reaction mixture may be avoided by separating the reaction product from the excess silica-containing material and/or from the insoluble impurities by means of filtering elements arranged towards the bottom of the reactor, the filtration process advantageously being carried out under temperature and pressure conditions similar to those prevailing during the preceding hydrothermal synthesis. The total reaction time is about 8 to 9 h.
Published European patent application No. 33,108, which is commonly assigned with this invention, describes a hydrothermal sodium silicate solution production process in which sand and NaOH aqueous solution are reacted at 150.degree.-200.degree. C. and corresponding pressure equivalent to that of saturated steam. The reaction is conducted in a cylindrical autoclave rotating about its horizontal axis, and the unreacted excess sand is used as a partial filter medium for the produced solution before being recycled to the production reaction. The produced solution has a SiO.sub.2 :NaOH weight ratio of 1-2.8:1. The undissolved fines are separated outside the reactor after expansion and cooling of the reaction mixture to around 100.degree. C. This different technical concept has a considerable bearing not only upon the structure of the plant itself, but also upon the potential volume/time yield. Whereas the process according to Belgian Pat. No. 649,739 requires a reaction time of about 4 hours and an at least equally long non-productive time, particularly for filtration inside the reactor, the process according to published European patent application No. 33,108 requires a reaction time of only 60 to 90 minutes and a non-productive time of around 30 minutes.
The reaction time required according to published European patent application No. 33,108 is still too long for a cost-optimal hydrothermal process, competitive with the above-mentioned melting/dissolving process, for the production of clear sodium silicate solutions from sand and waterglass with a ratio by weight of SiO.sub.2 to Na.sub.2 O of the order of 2:1. In addition, a large excess of sand in the reaction mixture necessitates recycling of the unused sand after filtration which incurs additional costs.